The invention relates to improvements in paper machine headboxes, and more particularly to improvements in the slice chamber of headboxes wherein trailing elements extend freely toward the slice opening for maintaining fine scale turbulence in the stock at the slice opening.
The concept of providing a freely movable self-positionable trailing element in the slice chamber of a headbox was first disclosed in U.S. Pat. No. 3,939,037, Hill. In U.S. Pat. No. Re. 28,269, Hill et al, trailing elements are disclosed extending pondside to pondside. These trailing elements are capable of generating or maintaining fine scale turbulence in the paper stock flowing toward and through the slice opening. The concepts of the foregoing patents may also be employed to utilize their advantage and to function in a machine for making multi-ply paper wherein stocks of different characteristics are fed to chambers on opposite sides of the trailing elements where the elements extend pondside to pondside.
A basic limitation in headbox design has been that the means for generating turbulence in fiber suspension in order to disperse the fibers have been only comparatively large-scale devices. With such devices, it is possible to develop small scale turbulence by increasing the intensity of turbulence generated. Thus, the turbulence energy is transferred naturally from large to small scales and the higher the intensity, the greater the rate of energy transfer and hence, the smaller the scales of turbulence sustained. However, a detrimental effect also ensued from this high intensity large-scale turbulence, namely, the large waves and free surface disturbance developed on the Fourdrinier table. Thus, a general rule of headbox performance has been that the degree of dispersion and level of turbulence in the headbox discharge was closely correlated; the higher the turbulence, the better the dispersion.
In selecting a headbox design under this limiting condition then, one could choose at the extreme, either a design that produces a highly turbulent, well-dispersed discharge, or one that produces a low-turbulent, poorly dispersed discharge. Since either a very high level of turbulence or a very low level (and consequent poor dispersion) produces defects in sheet formation on the Fourdrinier machine, the art of the headbox design has consisted of making a suitable compromise between these two extremes. That is, a primary objective of the headbox design up to that time had been to generate a level of turbulence which was high enough for dispersion, but low enough to avoid free surface defects during the formation period. It will be appreciated that the best compromise would be different for different types of papermaking furnishes, consistencies, Fourdrinier table design, machine design, machine speed etc. Furthermore, because these compromises always sacrifice the best possible dispersion and/or the best possible flow pattern on the Fourdrinier wire, it is deemed that there is a great potential for improvement in headbox design today.
The unique and novel combination of elements of the aforementioned patents provide for delivery of the stock slurry to a forming surface of a papermaking machine having a high degree of fiber dispersion with a low level of turbulence in the discharge jet. Under these conditions, a fine scale dispersion of the fibers is produced which will not deteriorate to the extent that occures in the turbulence dispersion which are produced by conventional headbox designs. It has been found that is the absence of large-scale turbulence which precludes the gross reflocculation of the fibers since flocculation is predominately a consequence of small scale turbulence decay and the persistence of the large scales. Sustaining the dispersion in the flow on the Fourdrinier wire then, leads directly to improved formation.
The method by which the above is accomplished, that is, to produce fine scale turbulence without large scale eddies, is to pass the fiber suspension through a system of parallel cross machine channels of uniform small size but large in percentage open area. Both of these conditions, uniform small channel size and large exit percentage open area, are necessary. Thus, the largest scales of turbulence developed in the channel flow have the same order of size as the depth of the individual channels by maintaining the individual channel depth small, the resulting scale of turbulence will be small. It is necessary to have a large exit percentage open area to prevent the development of large scales of turbulence in the zone of discharge. That is, large solid areas between the channel's exits, would result in large-scale turbulence in the wake of these areas.
In concept then, the flow channel must change from a large entrance to a small exit size. This change should occur over a sustantial distance to allow time for the large-scale coarse flow disturbances generated in the wake of the entrance structure to be degraded to the small-scale turbulence desired. The area between channels approaches the small dimension that it must have at the exit end. This concept of simultaneous convergence is an important concept of design of this invention.
Under certain operating conditions, the trailing members which are employed to obtain the fine scale turbulence are not necessarily stable. Cross-machine transient pressures tend to bend the trailing element in the cross-machine direction and cause cross-machine uniformity variances in the paper. Resistance to deformation along the machine direction length of the trailing elements can cause slight digressioins in the uniform velocity of the stock flowing off the surfaces at the trailing edge of the trailing element. Static or dynamic instability can occur at certain operating conditions and resonant frequencies can be reached dependent on the hydrodynamic forces. It has been discovered that the inertia and hydrodynamic couplings can be broken by suitable distribution of the mass and elasticity of the trailing structure with proper mass distribution and stiffness distribution being of importance.
It is accordingly an object of the invention to provide an improved trailing element design which avoids disadvantages that occur at certain operating conditions in structures heretofore available, and particularly a trailing element which offers resistance to a deflection in the cross-machine direction and which offers minimal resistance to deformation in the fluid flow stream so that pressures are balanced on opposite sides of the trailing edge of the trailing elements.
Definition of Terms:
machine direction: Flow direction. PA1 isotropic: Having the same properties in all directions. PA1 anisotropic: Not isotropic, that is exhibiting different properties when tested along axes in different directions.
In accordance with the principles of the invention, the objectives are attained by providing a trailing element which has a greater structural stiffness (preferably at the downstream tip) in the cross-machine direction than in the machine direction, and in a preferred form which is made of an anisotropic material, preferably on being formed of a laminate with separate layers of the laminate providing the qualities of cross-machine stiffness and machine direction strength and flexibility by either material properties, direction, size or number. Alternates of woven or needled material with weave directions or materials, or size or numbers of filaments controlling directional stiffness may be used.
By utilizing an anisotropic material, design factors which are otherwise not alway available can be included such as strength, stiffness, corrosion resistance, wear resistance, weight, fatigue life, thermal expansion or contraction, thermal insulation, thermal conductivity, acoustical insulation, damping of vibrations, buckling, low friction and optimal design in manufacture.
Other objects, advantages and features will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiment in the specification, claims and drawings, in which: